Field of the Invention (Technical Field)
Embodiments of the present invention relate to methods and apparatuses for template-dependent amplification of nucleic acid target sequences by oscillating reaction temperature in a small range, preferably no more than 20° C. during any given thermal polymerization cycle.
Background Art
Note that the following discussion refers to a number of publications and references. Discussion of such publications herein is given for more complete background of the scientific principles and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Amplification of nucleic acids is among the most indispensible techniques in molecular biology, widely used in research, genetic testing, agriculture, and forensics. The most common amplification method is the polymerase chain reaction (PCR) in which the prevalence of a specific nucleic acid target sequence is increased exponentially in solution (See U.S. Pat. Nos. 4,683,195, 4,683,202, and 4,800,159). A PCR reaction employs two oligonucleotide primers that hybridize to opposite strands of the DNA double helix either upstream (5′) or downstream (3′) of the target sequence to be amplified. A (generally thermostable) DNA polymerase is used to extend hybridized primers in the 5′→3′ direction by adding deoxynucleoside-triphosphates (dNTPs) in order to ‘copy’ the target sequence and generate double-stranded DNA products. By cycling the temperature of the reaction mixture (typically 95° C. Celsius), the two strands of DNA can be separated at high temperature allowing them to serve as templates for further primer binding and polymerization at lower temperatures (e.g. 55° C. and 60° C.). After repeating this process many times, a single target sequence can be amplified into billions of copies.
While PCR is the gold-standard amplification methodology in the well-equipped laboratory, it is rather complex, requiring both expensive and sophisticated thermal cycling instrumentation with active heating and cooling heating elements and precise temperature control, and trained technicians to gather meaningful results. For instance, most PCR reaction requires rapid and precise cycling between at least two temperatures (e.g. 95 deg and 57 deg), that typically results in the use of an expensive and energy-inefficient Peltier engine (thermal electric cooling mechanism) and precise temperature control elements. Such inherent limitations make PCR incompatible with the development of cost-effective, point-of-care nucleic acid diagnostics—useful where a supporting laboratory infrastructure is absent. In an effort to eliminate some of the resource-intensive requirements of PCR, various ‘isothermal’ amplification techniques have been developed in the past decades. In such reactions, nucleic acids may be amplified at a single temperature, removing the requirement of the costly thermocycler, and making them more amenable for use in low-cost diagnostic devices. Examples include nucleic acid sequence-based amplification (NASBA), helicase-mediated amplification, strand displacement amplification, loop-mediated isothermal amplification (LAMP) etc. However, these isothermal amplification methods often require 60-90 minute amplification time (due to slow kinetic enzymatic process in vitro) and precise temperature control at the single temperature point to accommodate the extremely stringent amplification reactions, again lacking the robustness and speed desired for the point of use diagnostic application.
Template-dependent nucleic acid amplification is the cornerstone of the nucleic acid-based molecular diagnostics. Robust, low cost, rapid, point-of-care nucleic acid diagnostics are a pressing need in health care, agriculture, and in the context of biological terrorism and warfare. However, the assay chemistry strategies associated with the existing PCR or isothermal amplification posse significant engineering and robustness limitations rendering such amplification approaches expensive and impractical for the resource-limited settings where nucleic acid-based molecular could make the most impact for emerging disease prevention and control. Considerable improvements in nucleic acid amplification must yet be made to bring affordable and robust diagnostics to settings devoid of dedicated laboratory infrastructure.
Conventional PCR relies on highly specific and rapid thermal cycling, commonly varying temperature by as much as 40° C. Such an amplification methodology requires expensive instrumentation in order to rapidly heat and (particularly) cool the PCR reaction mixture, in addition to accurately maintaining solution temperatures. Isothermal nucleic acid amplification procedures, while eliminating the need for complex thermal cycling instrumentation, are generally slow (at least 60 minute reaction time), unreliable, and require precise temperature calibration.
In a PCR thermal cycling process, a PCR cycler must have a good temperature control to maintains temperature uniformity within the sample and a typical sample heating (and/or cooling) rate of at least 2° C. per second. Temperature control is typically achieved by a feedback loop system, while temperature uniformity is achieved by highly thermally conductive but bulky materials such as copper. A high heating rate is accomplished by the implementation of a proportional integrated derivative (PID) control method limited by maximum dissipated power and heat capacitance. A high cooling rate is rather difficult to achieve and bulky systems require forced cooling by either a thermoelectric element (P. Wilding, M. A. Shoffner and L. J. Kricka, Clin. Chem., 1994, 40, 1815-1817.)(often called a Peltier element) or by other means, such as water (J. B. Findlay, S. M. Atwood, L. Bergmeyer, J. Chemelli, K. Christy, T. Cummins, W. Donish, T. Ekeze, J. Falvo, D. Patterson, J. Puskas, J. Quenin, J. Shah, D. Sharkey, J. W. H. Sutherland, R. Sutton, H. Warren and J. Wellman, Clin. Chem., 1993, 39, 9, 1927-1933). These PCR machines are complicated and power hungry devices. As the systems are bulky, their thermal time constants are in minutes rather than seconds which result in long transition times and unwanted by-products of the PCR. The high power consumption eliminates the possibility of making a battery operated and portable PCR system.
With the recent advancement of silicon technology based micromachining and biological micro-electromechanical systems (bioMEMS), many groups around the world have started the development of microPCRs (μPCR), which are a central part of a lab-on-a-chip or micro total analysis systems (μTAS). Researchers follow two basic approaches: a stationary system with cycling temperature a flow system with three zones at different temperatures. Both systems have their advantages and disadvantages. Stationary systems cycle the temperature of the chamber in order to modify the temperature of the PCR solution. They do not require a pumping system or other means to move the PCR sample around. The flow-through systems typically have zones at three constant temperatures. Only the sample changes temperature by moving between zones of different temperatures. This type of PCR system is faster than the first one but it requires an implementation of a mechanism to move the sample around. In both cases, the heaters are integrated with the PCR system, so it is not economical to dispose the device to avoid cross-contamination after performing only a single test. The major advantages demonstrated by these two formats are reduced cycle time with the use of reduced sample volume compared to a conventional device. However, these PCR chips use substrate materials such as silicon that require the employment of expensive and sophisticated fabrication process, leading to a very high unit price. Furthermore, as a result of extreme small reaction volume (<μl) to achieve increased surface to volume ratio and the type of materials used in the μPCR chips, some effects not very common with the conventional PCRs become significant, including nonspecific adsorption of biological samples, inhibition, sample evaporation, and formation of bubbles. Other current effort also involves the development of a temperature cycling reaction microchip that integrates stationary chamber and continuous flow PCRs to perform efficient temperature cycling of the flow-through microchannel PCR chip while the flexibility of varying the cycle number and the number of temperature zones in the stationary chamber PCR chip. However, the efficiency of the hybrid PCR device is still being validated and issues related to sample inhibition, adsorption, and bubble formation associated with such μPCR chip approach remain to poses significant stringency to all the upfront sample preparation/nucleic acid isolation process, and amplification reagents and reaction conditions e.g. ultra high polymerase concentration, PCR primer concentrations etc.